1. Field of the Invention
The present invention relates to speech coding/decoding, and more particularly, to an apparatus, method, and medium for reproducing a scalable wide-band speech signal.
2. Description of the Related Art
With the increased amount of speech communication applications in various fields, and an increase of network transmission speeds, there is an emerging demand for high fidelity speech communication. Accordingly, wide-band speech signals in the range of 0.05 kHz to 7 kHz, which show excellent capability in terms of naturalness and intelligibility in comparison with a known speech communication band ranging from 0.3 kHz to 3.4 kHz, are required to be transmitted.
In a packet switching network in which data is transmitted in unit of packets, a channel bottleneck may be caused, which may lead to packet loss and poor speech quality. Although a technique for hiding packet damage is known, this is not a satisfactory solution. Thus, a technique for scalable coding/decoding a wide-band speech signal has been proposed in which the wide-band speech signal can be effectively compressed, and the channel bottleneck can be reduced. Currently proposed methods of coding/decoding wide-band speech signals include a method in which speech signals in the range of 0.05 kHz to 7 kHz are simultaneously compressed and then restored, and a method in which speech signals are hierarchically compressed by being divided into signals in the range of 0.05 kHz to 4 kHz and signals in the range of 4 kHz to 7 kHz, and then restored. The latter method above is a wide-band speech coding/decoding method using a bandwidth scalability function for enabling optimum communication under the given channel condition by controlling the size of layers to be transmitted according to a data bottleneck condition. In the speech coding method using a bandwidth scalability function, a speech signal is coded and decoded using a hierarchical coding method. That is, the speech signal is coded after being divided into a core layer and a speech enhancement layer. The core layer transmits only information capable of restoring a minimum speech quality. The speech enhancement layer transmits additional information capable of enhancing speech quality. A method for providing a bandwidth scalability function in order to enhance speech quality is disclosed in U.S. Pat. No. 5,455,888, which is incorporated by reference in its entirety. FIG. 1 is a block diagram of a conventional bandwidth extension speech coding apparatus used in U.S. Pat. No. 5,455,888. FIG. 2 is a block diagram of a convention bandwidth extension speech coding apparatus used in U.S. Pat. No. 6,895,375, which is incorporated by reference in its entirety. In the conventional bandwidth extension speech coding apparatuses illustrated in FIGS. 1 and 2, information on a spectral shape and a power gain is used so that a power level is adjusted by using the power gain less than a spectral envelope that shows the spectral shape.
However, if a high-band speech signal is coded using conventional methods, the speech signal cannot be easily restored with high fidelity when the speech signal is transmitted at a low bit-rate. Further, the lower the bit-rate, the poorer the speech restoring capability. In addition, the conventional methods have not provided scalable wide-band speech reproduction for reducing/eliminating the channel bottleneck.